The present invention relates the medical diagnostic arts. It finds particular application in connection with monitoring of the speed of rotation of a rotating anode in an x-ray source, and will be described in conjunction therewith. It should be appreciated, however, that the invention is also applicable to the measurement of the rotation speed of other rotating bodies.
X-ray sources, such as those utilized in the field of medicine for the imaging of subjects, frequently employ a rotating anode, which is bombarded by a beam of electrons from a thermionic filament cathode. A heating current, commonly of the order of 2 to 5 amps, is applied through the filament to create a surrounding electron cloud. A high potential, of about 100 to 200 kilovolts, is applied between the filament cathode and the anode to accelerate the electrons from the cloud towards the anode. The beam of electrons is directed to a focal track on an inclined, annular surface or target area of the anode. X-radiation radiates in response to the impingement of the electrons on the target area.
The acceleration of electrons causes a tube or anode current of about 500-600 milliamps. Only a small fraction of the energy of the electron beam is converted into x-rays, the majority of the energy being converted to heat which heats the anode white hot. The temperature of the anode can be as high as about 1,400xc2x0 C. In high energy tubes, therefore, the anode rotates at high speeds during x-ray generation to spread the heat energy over a large area and to inhibit the target area from overheating. The cathode and the envelope remain stationary. Due to the rotation of the anode, the electron beam does not dwell on the small impingement spot of the anode long enough to cause thermal deformation. The diameter of the anode is sufficiently large that in one rotation of the anode, each spot on the target area that was heated by the electron beam has substantially cooled before returning to be heated by the electron beam.
The anode is typically rotated by an induction motor. The induction motor includes driving coils, which are placed outside the glass envelope, and a rotor with an armature and a bearing shaft, within the envelope. The armature and/or bearing shaft is connected to the anode. When the motor is energized, the driving coils induce electric currents and magnetic fields in the armature which cause the armature and hence the target area of the anode to rotate.
For maximum useful life of the X-ray source, it is important to maintain the rotational speed of the anode at, or close to, a predetermined value. If the anode rotation speed drops too low, thermal damage to the target area can result. High anode rotation speeds, on the other hand, result in the stator motor operating more than is needed, and can lead to thermal damage. Whenever the motor is running, heat is generated and is transferred to the x-ray tube housing. It is also undesirable for the source to be operated at the rotation speed of mechanical resonance of the anode and the rotor. Additionally, on start-up, it is preferable to delay application of the power to the cathode for generation of electrons until the anode has reached a minimum rotation speed. Accordingly, it is important to be able to measure the speed of rotation of the anode and to be able to make adjustments, if needed, in response to the detected speed.
Various detectors have been developed to ensure that the anode is rotating at its design operation speed. In one design, bearing shaft rotation is detected. For example, an optical feed-through with a fiber optic source is used to detect the movement of an optically readable timing marker fitted to the bearing shaft of the rotor. Devices which measure bearing shaft angle rotation, however, typically involve the installation of an optical, mechanical, or electrically responsive device along the shaft itself, which, in the case of an x-ray source, invades the housing of the source in order to install such a detection device.
In another design, the power to the stator is shut off momentarily, and the back EMF generated by the spinning rotor is measured across the stator. This results in a drop in rotation speed each time the speed is measured.
Devices have been developed which make use of naturally occurring defects in the target area to determine rotation speed. However, these employ complex analytical equipment to compensate for the irregularities of the defects and their uneven spacing on the target.
Lasers have been used as an indirect measurement of the rotational speed. An externally generated laser beam is reflected off the target and used to measure the temperature. The temperature of the target area is dependent on the rotation speed, and thus the measured temperature gives an indirect indication of speed. However, this method does not facilitate correction of the rotation speed. The anode takes a finite time to cool or heat up when the speed is increased or decreased, and thus over-correction may occur.
The present invention provides a new and improved apparatus and method of monitoring the speed of rotation of an anode, which overcomes the above-referenced problems and others.
In accordance with one aspect of the present invention, a detection system for detecting the rotational speed of an anode of an x-ray tube is provided. The x-ray tube includes a first source of electrons which are accelerated at a target area of an anode to generate a primary x-ray beam. The detection system includes a second source of electrons which are accelerated at the anode to generate a second x-ray beam. A defect on the anode periodically changes an x-ray distribution of the second x-ray beam at least along a detection direction. An x-ray detector detects an intensity of the second x-ray beam along the detection direction.
In accordance with another aspect of the present invention, an x-ray tube is provided. The x-ray tube includes an evacuated envelope and an anode rotatably mounted in the evacuated envelope. The anode has a circular primary target area around a periphery of the anode and an inner circular track of smaller radius than the primary target area. The anode has a construction along the inner track that alters a distribution of generated x-rays. A first cathode cup is mounted within the evacuated envelope for generating electrons that are accelerated into the primary target area to generate a primary x-ray beam. A second cathode cup is mounted within the evacuated envelope for generating electrons that are accelerated at the inner track to generate a secondary x-ray beam. An x-ray distribution of the secondary beam changes each time the accelerated electrons strike the construction. An x-ray detector is positioned to monitor the changes in the secondary beam distribution as the electrons strike the construction. A motor rotates the anode.
In accordance with another aspect of the present invention, a method for determining rotational speed of a rotating anode of an x-ray source is provided. The x-ray source includes a first source of electrons which are directed at a rotatable anode to generate a primary x-ray beam. The method includes providing the anode with a defect in a surface thereof, rotating the anode, and, while the anode is rotating, directing electrons at the anode from a second source of electrons to generate a secondary beam of x-rays. The intensity of the secondary beam of x-rays along a detection direction changes as the defect interacts with the electrons from the second source of electrons. the method further includes determining a rotation speed of the anode from a frequency at which the intensity of the secondary beam of x-rays changes in response to the interaction of the electrons from the second source with the defect.
One advantage of the present invention is that the speed of a rotating x-ray anode is measured.
Another advantage of the present invention is that it enables correction of the rotation speed of the anode in response to the detected rotation speed.
Another advantage of the present invention is that it enables an x-ray tube to be operated at optimum efficiency for a longer useful life.
Another advantage of the present invention is that it enables measurement of anode rotation speed and generation of x-rays to be carried out simultaneously.
Another advantage of the present invention is that it avoids the use of complex analytical equipment for determining anode rotation speed.
Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following data ed description of the preferred embodiments.